This invention relates generally to time measurement devices and particularly to devices for making parallel measurements of the times when different phenomena occur.
In an automatic focus camera (AFIC), the image of an object is formed on two light receiving element or photosensor arrays and the distance between the camera and the object is determined from the difference between the positions of the images on those arrays. To make that determination, some photosensor arrays use a system to measure photosensor response times and generate electrical signals representing the images. Many systems of this type are well known in the art.
FIG. 1 is a block diagram of a conventional time measuring system 10 which includes AND gates 12 (12a through 12n) and counters 13 (13a through 13n) to measure the times required for the outputs of photosensors 11 (11a through 11n) to reach a predetermined level. For example, if the outputs of photosensors 11 change from a high (H) level to a low (L) level when a desired response is sensed, counters 13 count the number of clock signals .phi..sub.0 generated while the outputs of photosensor 11 remain at a high level. The response times of photosensors 11 thus correspond to the count values of counters 13.
With some modifications, system 10 can also be adapted to measure the time elapsed between the onsets of selected phenomena. Conventional time measurement systems, however, have inherent limitations.
In conventional systems for measuring the onset times of different phenomena, the amount of hardware increases not only with the number of phenomena to be measured but also with the length of the time durations to be measured. For instance, the measurement of a one second time duration with a 1 MHz clock signal requires a twenty stage binary counter (10.sup.6 is approximately equal to 2.sup.20). In addition to the complexity of such systems, time measurement which involves an excessively large number of digits delays succeeding data processing operations and hinders the production of sufficiently effective results. This is even more of a disadvantage when the precision afforded by all the digits is not needed.
A solution to these problems of complexity and loss of effectiveness of conventional time measurement systems is premised on the realization that photosensor response times do not always need to be measured very precisely. Especially when the outputs of the sensors are quantized, often only the most significant data from the sensor arrays are needed for the the time measurement system to operate effectively.
Accordingly, an object of the invention is to provide a time measuring device which can perform effective time measurement with a relatively small amount of hardware.